If you want stories about a couple in a relationship (that don't end at the confession of love):
Clannad: After Story
Please Teacher
Say "I Love You"
Mysterious Girlfriend X
Aishiteruze Baby (takes about half of it for romance to get going)
Love, Chunibyo & Other Delusions (2nd season in particular)
Itazaru na Kiss
Monogatari Series

If you want stories that mix a bit of supernatural into a solid romance:
Anohana
Clannad
Kanon (2006)
Natsuyuki Rendezvous
Dusk Maiden of Amnesia
Kotoura-San
Monogatari Series